1. Field of the Invention
The invention is concerned with fabrics, which are particularly, but not exclusively, for use as papermachine clothing and ideally for use in the forming section of the papermachine.
2. Description of the Related Art
The original synthetic forming fabrics utilised one warp system and one weft systems. However these, so called, single-layer fabrics tended to narrow and stretch thus interfering with paper production and machine performance. So called mlti-layer fabrics were therefore developed to overcome the dimensional instability of the single-layer fabrics. Of these, the so called two-layer fabrics have become the most common type of forming fabric. Two-layer fabrics utilise one warp system which interlaces with two distinct weft systems. The warp system provides a greater amount of warp material, for stability, than the prior single-layer fabrics.
Two-layer weaves, in addition to improving fabric stability, also allow further benefits over single-layer fabrics. These benefits are due to the two separate weft systems.
The wearside weft system protects the load-bearing warp yarns by providing long "floats" which contact the dewatering elements on the papermachine. Furthermore as the wearside weft never appears on the paperside then relatively thick yarns can be used to provide a significant amount of material for wear. By this means improved fabric lives can be obtained without causing undesirable wiremark.
The paperside weft system interlaces with the warp to provide a surface for the formation, dewatering and release of the papersheet. Relatively thin weft yarns may be used to minimise wiremark, as these yarns do not appear on the wearside. Improved papermaking properties are thus obtained.
Single-layer fabrics were subsequently made using weave patterns which also offered inproved dimensional stability. For example U.S. Pat. No. 4,518,644 teaches that relatively stable single-layer fabrics can be obtained by causing "the longitudinal and/or transverse threads to change the direction of their course intermittently in the plane of the fabric". The resulting lateral crimp added a pronounced "diagonal" aspect to the usual three yarn orientations of: machine direction, cross-machine direction, and vertical crimp. The lateral crimp was of sufficient magnitude that adjacent yarns made intermittent contact such that a buttressing effect was achieved.
Improvements in single-layer stability were possible by this means. However as such structures still possess only single weft and warp systems it is not possible to obtain both the papermaking and wear resistance improvements which are provided with the two-layer structures.
Furthermore, U.S. Pat. No. 4,518,644 had the declared objective of obtaining a single-layer fabric with maximum thickness, maximum inner volume and a minimum open area (in vertical sight). It is now known that the foyer two properties can contribute to water carrying within the fabric body.
Fabric water carry is particularly undesirable on Gap Former machines. Such machines operate at relatively high speed and possess comparatively short dewatering sections with which to remove water from the sheet. Consequently such machines tend to produce sheets with an undesirably high moisture content. This problem is worsened by moisture carried within the voids of the fabric rewetting the sheet. Bearing this in mind an ideal structure for use on modern papermachines will possess relatively low thickness and void volume.
The single-layer structure described in U.S. Pat. No. 4,518,644 also maintained a high permeability indicating the relative openness of the structure and the limitation of the structure with regards to retention of fines and filler.
Latterly a need for paper with, for example, improved printing characteristics developed. It was realized that such paper could be produced by using two-layer fabrics with an increased paperside: wearside weft ratio of 2:1. Thus EP 0085363 allowed for the inclusion of an additional set of "floater" weft yarns on the cloth paperside to improve retention and papermaking characteristics.
The set of floater weft yarns described in EP 0085363 are of "substantially" smaller diameter than the set of parallel integral paperside weft yarns with which the floater yarns alternate. Preferably the diameter of the floater yarns is 50-75% that of the interwoven parallel yarns.
The floater wefts make no interlacing with the warp in contrast to the integral weft yarns which do interlace with the warp. As a consequence of the difference in crimp patterns and weft diameters between the two sets of paperside weft yarns it is not possible for the two sets of wefts to achieve an approximately level height on the paperside when material possessing identical properties is used for both sets of weft.
An ideal fabric will provide a good papermaking surface by means of numerous regularly distributed support points. To offer useful primary support to the fibre mat or papersheet these support points must be of similar height. Consequently the structures described in EP 0085363 must necessarily utilize material of significantly different thermal shrinkage to allow the two sets of paperside weft yarns to sit at approximately the same height on the paperside surface of the fabric.
A further drawback with the structures formed according to EP 0085363 is their relative instability. Such structures are relatively unstable because the thin "floater" wefts, which in two-layer structures typically represent one-third of all of the weft yarns present, make no interlacings with the warp yarns.
Consequently such structures contain a comparatively low number of yarn interlacings thus allowing adjacent warp yarns to shift position with relative ease. This type of shearing movement is indicative of fabric structures which may narrow significantly on a misaligned or irregularly worn papermachine.
Fabric narrowing will result in warp density variation which in turn may cause uneven drainage across the width of the fabric. Such fabric may also be prone to rippling in the machine direction. This effect is particularly problematic on Gap Former type machines which utilise an enclosed forming zone and are prone to "streaky" sheet formation in the machine direction. An ideal forming fabric will, therefore, possess a structure with a high resistance to shear distortion.
U.S. Pat. No. 4,739,803 discloses a two-layer fabric with a weft ratio of 2:1 wherein all of the weft yarns are interlaced with warp yarns. Thus shear resistance may be enhanced. However, as with the prior type of 2:1 fabric with floater wefts, there are still alternating sets of wefts on the paperside of this fabric. The first set of wefts are supported in a crimp "saddle" whereas the second set of wefts are supported in a shear like manner between warp yarns. Thus two sets of support points are created at the fabric paperside. The resulting sets of support points tend to sit at different heights.
To compensate for the difference in height of paperside support points of cloth according to U.S. Pat. No. 4,739,803, and thus optimise sheet support for the structure, it is necessary for the two sets of paperside weft yarns to be of different diameter and/or for the two sets of paperside weft yarns to be of different material and/or thermal shrinkage. Raw material and production processes must thus be strictly controlled to obtain the desired fabric.
As has been stated to minimise sheet rewetting it is desirable to avoid both relatively high void volume and high fabric thickness. However both of the prior art two-layer designs so far described maintain relatively high void volume and thickness.
Although the structure described in U.S. Pat. No. 4,739,803 develops a degree of lateral crimp in at least some o the paperside weft yarns no weft buttressing is described therein.
Additionally both prior art structures maintain a high permeability.
The air permeability of a fabric can indicate the openness of the paperside surface. A highly open fabric surface can promote excessive fibre penetration resulting in sheet sealing of the fabric drainage channels. An undesirably rough paper surface may therefore occur.
Furthermore the subsequent dewatering efficiency of the structure can be reduced by this effect.
It is also understood that fabric with high air permeability may allow similarly high initial dewatering of the fibre stock. The fast initial dewatering can cause low retention due to the fines present in the stock being washed out. Additionally a compact layer of fibre at the fabric surface may be created through which it becomes difficult to remove the remaining water.
By reducing the openness of the fabric's paperside surface, as may be indicated by a lower fabric permeability, it is possible to reduce the rapid rate of initial drainage. The controlled initial sheet dewatering allows a more effective use of all the dewatering elements positioned throughout the forming section such that water is removed more evenly.
The delayed dewatering facilitates the "working" of the fibres by dewatering elements such that good sheet formation can be achieved. This type of phenomena was certainly observed when the very high pemeability single-layer fabrics were superceded by the current double-layer structures.
However even the prior art double-layer structures may be considered to possess an unnecessarily high permeability.
A two-layer, 8-shaft weave repeat structure, with a weft ratio of 2:1 wherein all paperside weft yarns interlace in the same manner with the warp yarns is described in GB 2245006A.
The object of the invention described therein is to obtain a stable structure. This is sought by employing a short weave repeat such that the warp yarns must "rise and descend at a sharper angle when passing between the upper and lower surfaces of the fabrics". Thus GB 2245006A is unlike the current invention which utilises the buttressing action of adjacent weft yarns to obtain excellent shear resistance.
As with all other prior art structures a high permeability fabric is obtained. In fact permeability is stated as being "extremely high as compared with prior art solutions".
It is intimated that for good papermaking properties the density of weft yarns in the paperside of the fabric is set to achieve an air permeability of at least 500 c.f.m. In this respect also the fabric described in GB 2245006A is drastically different from that invented by the applicants.
The concept of a high permeability structure with a relatively open surface is diametrically opposed to that pertaining to the invention.